"Since the contract was concluded long ago, naturally that it's impossible to just dust it off in the form it has been preserved," Ryabkov said.

"I can say that except adapting the contract for being implemented right now, we should clarify the situation with Iran’s lawsuit against the Russian side for improper fulfilment of that contract," he said. "We expect the problem to be solved at the bilateral agenda."

"I cannot speak for Iran but I am ready to say that we are interested in solving this issue," he said.

Russia's 2007 contract on S-300 deliveries to Iran

Under the 2007 contract, Moscow was to deliver to Tehran five divisions of the missile systems of medium range worth over $800 million.

The Iranian side paid $166.8 million in advance. However, until mid-2010 the systems were not supplied to Iran.

In September 2010, then-President Dmitry Medvedev signed a decree on measures on implementing the UN Security Council’s resolution 1929 that in particular banned the S-300 supplies to Iran.

The contract was canceled and the advance payment was sent back to the Islamic Republic.

Russia lifts ban on air defense system delivery to Tehran

Russian President Vladimir Putin signed a decree allowing S-300 deliveries to Iran on April 13. The document came into force on the day it was signed.

Russian Foreign Minister Sergey Lavrov said Russia’s voluntary embargo on deliveries of S-300 missile systems to Iran is no longer needed due to progress in the resolution of the situation around Iran’s nuclear program.

"Initially, the decision to suspend the implementation of the contract, which was already signed and came into force, was made in September 2010," he recalled. "It was done in the interests of support for consolidated efforts of the six international negotiators to stimulate a maximally constructive process of talks on settlement of the situation around Iran’s nuclear program."

The minister particularly stressed that "it was done absolutely voluntarily."